Electromagnetic flowmeters measure the volumetric flow rate of an electrically conductive fluid flowing in a measuring tube. The part of the measuring tube which comes into contact with the fluid is electrically nonconductive, so that a voltage induced by a magnetic field according to Faraday's law of induction would not be short-circuited if the measuring tube were completely made of metal, i.e., if the fluid came into contact with the metal. The magnetic field generally cuts the measuring tube in a direction perpendicular to the longitudinal axis of the tube.
Therefore, metallic measuring tubes, which, of course, must not be ferromagnetic so as not to disturb the magnetic field, are commonly provided with an electrically non-conductive inner layer as insulation; in the case of plastic or ceramic measuring tubes, this non-conductive layer is not necessary.
The induced voltage is picked off by means of galvanic or capacitive measuring electrodes. Galvanic measuring electrodes extend through the wall of the measuring tube and thus contact the fluid, while capacitive measuring electrodes are so fitted in the wall of the measuring tube as not to contact the fluid, or are mounted on the wall of the measuring tube.
U.S. Pat. No. 5,402,685 discloses an arrangement for operating several electromagnetic flow sensors with a single electronic evaluating unit, said arrangement comprising the following parts, which are of interest in connection with the present invention:
a first measuring tube with two galvanic measuring electrodes and with an associated coil arrangement for producing a magnetic field; PA1 a second measuring tube with two galvanic measuring electrodes and with an associated coil arrangement for producing a magnetic field; PA1 a third measuring tube with two galvanic measuring electrodes and with an associated coil arrangement for producing a magnetic field; PA1 a penultimate measuring tube with two galvanic measuring electrodes and with an associated coil arrangement for producing a magnetic field; PA1 a last measuring tube with two galvanic measuring electrodes and with an associated coil arrangement for producing a magnetic field; PA1 a single processing unit for potentials of the measuring electrodes; PA1 a single generator circuit for producing a single coil current for the coil arrangements; and PA1 a switching arrangement for connecting the coil arrangement of a respective one of the measuring tubes to the generator circuit, and the measuring electrodes of said measuring tube to the processing unit. PA1 which comprises a measuring tube with two galvanic measuring electrodes and a coil arrangement for producing a magnetic field, PA1 said instrumentation amplifier arrangement comprising: PA1 a measuring tube with a first and a second galvanic measuring electrode; PA1 a coil arrangement for producing a magnetic field by means of a coil current; PA1 said instrumentation amplifier arrangement comprising: PA1 which comprises a measuring tube with at least two measuring electrodes and a ground electrode as well as a coil arrangement for producing a magnetic field, PA1 said instrumentation amplifier arrangement comprising per measuring electrode: PA1 said instrumentation amplifier arrangement further comprising: PA1 which comprises a measuring tube with at least two measuring electrodes and a ground electrode as well as a coil arrangement for producing a magnetic field, PA1 said instrumentation amplifier arrangement comprising:
As for the processing of the signals from the measuring electrodes, U.S. Pat. No. 5,402,685 merely includes a general reference to U.S. Pat. Nos. 4,210,022, 4,422,337, 4,382,387, and 4,704,908. All of these, however, describe measuring circuits without analog-to-digital converters.
U.S. Pat. No. 5,351,554 discloses an instrumentation amplifier arrangement of an electromagnetic flowmeter
a single differential stage having its input end connected to each of the measuring electrodes; PA2 an analog-to-digital converter following the differential stage; and PA2 a clock generator supplying a sampling signal to the analog-to-digital converter. PA2 a first impedance converter following the first measuring electrode; PA2 a second impedance converter following the second measuring electrode; PA2 a first double-throw switch having a first input and a second input connected to the output of the first impedance converter and to ground, respectively; PA2 a second double-throw switch having a first input and a second input connected to the output of the second impedance converter and to ground, respectively; PA2 an analog differential stage connected to an output of the first double-throw switch and to the output of the second double-throw switch; PA2 a further amplifier for a voltage developed across a resistor traversed by the coil current; PA2 a multiplexer connected to an output of the differential stage and to an output of the further amplifier; and PA2 an analog-to-digital converter following the multiplexer. PA2 a preamplifier connected to the measuring electrode at its input end; and PA2 an analog-to-digital converter connected directly to an output of the preamplifier; PA2 a clock generator supplying the analog-to-digital converters with a sampling signal PA2 a subtractor following the analog-to-digital converters. PA2 one preamplifier per measuring electrode; PA2 a first analog-to-digital converter connected directly to an output of one of the preamplifiers; PA2 a differential stage following the preamplifiers; PA2 a second analog-to-digital converter following the differential stage; PA2 a subtractor following the analog-to-digital converters; and PA2 at least one clock generator supplying the analog-to-digital converters with a sampling signal
In addition, this U.S. Pat. No. 5,351,554 mentions that in the circuit arrangement described therein, arrangements for compensating electrochemical interference voltages as are described in U.S. Pat. Nos. 4,210,022 and 4,704,908 can be used.
U.S. Pat. No. 5,370,000 describes an electromagnetic flowmeter comprising the following parts, which are of interest in connection with the present invention:
The prior-art instrumentation amplifier arrangements have proved effective many times. They operate with a coil-current value of, e.g., 85 mA. However, if fluids with high solids contents are to be measured, as is frequently necessary in the construction industry (stones in chuted concrete) or in the paper industry (wood fibers in paper stock), for example, solid particles striking the measuring electrodes will result in noise voltages, and thus in a degradation of the signal-to-noise ratio.
Attempts have been made to eliminate this degradation by increasing the coil current and, thus, the induced voltage picked off the measuring electrodes. However, this requires, among other things, more powerful power supplies, which, in turn, exhibit greater losses, so that total energy consumption increases. Also, larger electronics housings may be necessary.